Acid-phase protection of low-carbon steel by DIMC: evidenced by WL/PDP/EIS and density-functional modeling

Abstract

We synthesized and evaluated 2,2′-(2,3-dihydroxyterephthaloyl)bis(N-propylhydrazine-1-carbothioamide) (DIMC) as a corrosion inhibitor for low-carbon steel (LCS) in 0.5 M HCl. A multi-technique workflow—weight loss (WL), potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), and surface analysis (SEM/AFM)—shows concentration-dependent protection with maximum inhibition efficiency of 91.41% at 300 ppm and 298 K, remaining 80.24% at 328 K. EIS reveals two distinct time constants; refitting with R_s–(C_dl∥R_ct)–(Cfilm∥Rfilm) confirms robust charge-transfer suppression plus a film-relaxation response. DFT descriptors and Monte Carlo adsorption simulations corroborate mixed physisorption–chemisorption, with electron-rich N/S/O centers driving donor–acceptor interactions at Fe sites. Collectively, DIMC forms a stable adsorbed layer that mitigates both anodic and cathodic reactions in aggressive acid, positioning DIMC as a promising green inhibitor for acid-exposed steel systems.